Conventionally, an electric power tool such as an electric screwdriver or an impact wrench utilizes, as its power source, a battery pack detachably attached to a main body. Use of the battery pack makes it possible to eliminate a burden of handling, while at work, an electric cord electrically connected to an external power source, e.g., a commercial power source, and also makes it possible to perform a work at a job site where no external power source exists. If the electric power accumulated in a secondary battery built in the battery pack is reduced, the battery pack is detached from the electric power tool and attached to a battery charger connected to an external power source. The secondary battery is charged to a reusable level by the battery charger.
As shown in FIG. 6, a conventional battery charger 1 includes, e.g., an external connection unit 2 electrically connected to an external power source 16, a charging circuit for charging a secondary battery (not shown) built in a battery pack 17, a notification unit 12 having a light-emitting member such as a light-emitting diode (LED) or the like, and a central control unit 11, such as a microcomputer or the like, for performing the charging control of the charging circuit and the light-emitting control of the notification unit 12.
The charging circuit includes a transformer 6, a rectifying unit 3 for rectifying the electric current supplied from the external power source 16 through the external connection unit 2 and feeding the rectified current to the transformer 6, a switching device 4 for switching the transformer 6 and a switching control unit 5.
The rectifying unit 3, the switching device 4 and the switching control unit 5 are arranged at a primary side of the transformer 6. The charging circuit further includes a rectifying diode 7, a smoothening capacitor 8, an output voltage detecting unit 9 for detecting the smoothened secondary-side voltage and a charger terminal 10 for electrically interconnecting the internal circuit of the battery pack 17 and the charging circuit, all of which are arranged at a secondary side of the transformer 6. That is, the battery charger 1 performs the charging of the secondary battery by allowing the charging circuit to rectify an electric power supplied from the external power source 16 as an electric power source and boost a voltage and then feeding the rectified electric power to the internal circuit of the battery pack 17. The charging control of the charging circuit is performed by allowing the central control unit 11 to control the switching control unit 5 based on a detection result of the output voltage detecting unit 9 and the kind or charged voltage of the secondary battery outputted from the internal circuit of the battery pack 17.
As disclosed in Japanese Patent Application Publication No. 2003-9410, the charging operations of the charging circuit, such as the charging startup, the charging in progress and the charging completion, and the trouble occurring in the charging process, such as the heat generation in the secondary battery and the like, are differently notified to a user by turning on or flickering a notification unit or changing the color lights thereof, brightness or flickering period of a light-emitting member. Therefore, the user can easily figure out the charging state and trouble in a battery pack by merely seeing the indication on the notification unit. The notification unit is designed to notify respective operations only when the battery pack is attached to the battery charger. In a standby mode in which the battery charger is connected to the external power source but the battery pack is not attached to the battery charger, the notification unit is turned off to thereby reduce consumption of the electric power.
There is also known a battery charger of a type in which, when a battery pack is attached to the battery charger, a central control unit makes charging determination by which to determine whether a battery pack needs to be charged or not. Only the battery pack determined that it is needed to be charged is charged by a charging circuit. The charging determination made by the central control unit is performed based on a detection result of a voltage in the secondary battery built in the battery pack at a time when attaching. In a certain battery charger, charging determination is made depending on the presence or absence of an electric signal indicating that the detected voltage in a secondary battery is equal to or greater than a specified value, which means that the secondary battery is in a fully charged state. In the battery charger of the type that determines the necessity of performing a charging operation through the charging determination, no charging operation is performed with respect to a fully-charged battery pack, thereby preventing the secondary battery from being overcharged.
In case of the power saving type battery charger that turns off the notification unit in the standby mode, however, the user is not aware of trouble such as disconnection or breakage occurring in the battery chargers until the notification unit notifies the user of the occurrence of trouble upon attaching the battery pack or until the notification unit stays inoperable despite the attaching of the battery pack. In particular, even when the occurrence of trouble is notified by the notification unit, the user cannot know whether the trouble has occurred in the battery pack or in the battery charger, thereby making it inconvenient to use the battery charger.